The present invention relates to a rechargeable lithium electrochemical cell that is usable at low temperature.
A lithium electrochemical cell possesses an electrochemical stack including a positive electrode comprising electrochemically active material capable of inserting lithium into its structure (generally an oxide of a transition metal, usually lithiated), and a negative electrode that supplies the lithium ions. The electrodes are placed on either side of a separator membrane that is generally made of polyolefin. The electrochemical stack is impregnated in a non-aqueous electrolyte that is solid or liquid. The electrolyte contains a lithium salt dissolved in a mixture of organic solvents.
The low temperature operation of such cells has frequently been investigated. Attention has been given mainly to the composition of the electrolyte.
According to document JP-08 195 221, low temperature (xe2x88x9220xc2x0 C.) conductivity is increased when the solvent is made up of ethylene carbonate, propylene carbonate, an acetic ester, and at least one compound selected from diethyl or dimethyl carbonate, and dimethoxyethane. The volume fraction of the acetic ester is not more than 50%.
A rechargeable lithium electrochemical cell including an anode of metallic lithium or of lithium alloy, and a cathode whose active material is an electrically-conductive organic polymer, is described in document FR-2 641 130. The electrolyte is a lithium salt dissolved in a solvent, which salt is a combination of a cyclic carbonate and a non-cyclic carbonate. That cell retains sufficient discharge capacity at a temperature of less than 0xc2x0 C.
U.S. Pat. No. 4,056,663 describes a rechargeable electrochemical cell having a metallic lithium anode and a metal oxide cathode, in which the electrolyte comprises a solvent that does not contain ether. The solvent is a mixture of carbonates or a mixture of at least one carbonate and at least one ester.
According to U.S. Pat. No. 4,983,476, improved low temperature performance is obtained by replacing the metallic lithium of the anode with a transition metal sulfide. The electrolyte comprises an aprotic organic solvent such as methyl formate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, tetrahydrofuran, and mixtures thereof.
The most recent rechargeable lithium electrochemical cells possess a negative electrode of the paste type having a conductive support that acts as a current collector on which there is placed a paste containing a binder and an electrochemically active material which is a material that is capable of inserting lithium into its structure. The greater safety of such cells makes them suitable for a wider range of applications. For use in particular in an electric vehicle or in radiocommunications, such cells must be capable of operating at low temperature, and in particular at below xe2x88x9220xc2x0 C.
To this end, document EP-0 482 287 suggests using a rechargeable lithium electrochemical cell possessing a compressed anode containing carbon, a cathode comprising a lithiated oxide, and an electrolyte including a lithium salt dissolved in an organic solvent comprising a cyclic ester and a linear ester.
A rechargeable electrochemical cell having a carbon anode as described in document JP-09 022 738 contains an electrolyte whose solvent comprises a cyclic carbonate, a linear carbonate, and up to 44% by volume ethyl acetate. The binder of the electrodes is polyvinylidene fluoride, and the active material of the cathode is a lithium cobalt oxide. That cell has improved performance at low temperature. The document mentions a drop in performance at low temperature when methyl propionate or ethyl propionate is used as the solvent.
The discharged capacity of a rechargeable lithium electrochemical cell in accordance with document EP-0-531 617 can be increased. The cell comprises a carbon anode and a lithiated oxide cathode. The electrolyte solvent is a mixture of a cyclic carbonate, a linear carbonate, and a compound of formula RCOOR1, where R is an ethyl radical and R1 is an alkyl group having 1 to 3 carbon atoms. The respective volume fractions thereof are preferably 20% to 50%, 10% to 70%, and 10% to 70%.
Document EP-0 614 240 describes a rechargeable lithium electro-chemical cell having a carbon anode and a metal oxide cathode, with improved discharge at a high discharge rate, particularly at low temperature. The cell contains an electrolyte comprising a lithium salt and a mixture of aprotic solvents made up by volume of 10% to 20% ethylene carbonate, 5% to 40% propylene carbonate, and 50% to 85% dimethyl carbonate.
To improve high-rate discharge at low temperature, document EP-0 766 332 proposes an electrochemical cell comprising paste electrodes in which the binder is polyvinylidene fluoride (PVDF). It has an anode comprising an electrochemically active material based on carbon, and a cobalt oxide cathode. The solvent of the electrolyte is, by volume, made up of 50% to 60% of a mixture of cyclic carbonate and of cyclic ester, such as xcex3-butyrolactone or xcex3-valerolactone, 20% to 40% of a linear carbonate, and 10% to 25% of a linear ester.
Self-discharge during storage at low temperature is decreased in a rechargeable lithium electrochemical cell having a carbon anode, and a lithiated oxide cathode, in accordance with EP-0 548 449. The electrolyte solvent is a mixture of three components which are an aliphatic carboxylate, a cyclic carbonate, and a linear carbonate. The respective volume proportions thereof are preferably 10% to 80%, 20% to 50%, and not more than 70%.
An object of the present invention is to provide a rechargeable lithium electrochemical cell having a carbon anode in which performance during low temperature operation is better than that of known cells.
The present invention provides a rechargeable lithium electrochemical cell comprising an electrolyte containing a lithium salt dissolved in a non-aqueous solvent, at least one positive electrode, and at least one negative electrode of the paste type containing an electrochemically active material which is a carbon compound suitable for inserting lithium ions and a binder. The invention is characterized in that said solvent contains at least one saturated cyclic carbonate and at least one linear ester of a saturated aliphatic monocarboxylic acid, and in that said binder is a polymer having no fluorine.
The terms xe2x80x9clinear ester of a saturated aliphatic monocarboxylic acidxe2x80x9d and xe2x80x9csaturated aliphatic carboxylatexe2x80x9d are used to mean a compound of formula RCxe2x80x94Oxe2x80x94ORxe2x80x2 in which R is H or an alkyl group, and Rxe2x80x2 is an alkyl group such as CH3 (methyl), CH3xe2x80x94CH2 (ethyl), etc. . . . Said linear ester of a saturated aliphatic monocarboxylic acid is, for example, a formiate if R is H, an acetate if R is CH3, a propionate if R is CH3xe2x80x94CH2, a butyrate if R is CH3xe2x80x94(CH2)2, a valeriate if R is CH3xe2x80x94(CH2)3, etc. . . . .
In the solvent, the volume proportion of said saturated cyclic carbonate lies in the range 5% to 60% of said solvent, and the volume proportion of said linear ester lies in the range 20% to 85% of said solvent, the proportion of said linear ester is preferably not less than 50% of said solvent.
Said saturated cyclic carbonate is selected from propylene carbonate, ethylene carbonate, butylene carbonate, and mixtures thereof.
In a first variant, said saturated cyclic carbonate is ethylene carbonate.
In a second variant, said saturated cyclic carbonate is propylene carbonate.
In a third variant, said saturated cyclic carbonate is a mixture of ethylene carbonate and of propylene carbonate.
Said linear ester is selected from an acetate, a butyrate, a propionate, and mixtures thereof. By way of example, it is possible to select an ethyl acetate, a methyl acetate, a propyl acetate, an ethyl butyrate, a methyl butyrate, a propyle butyrate, an ethyl propionate, a methyl propionate, a propyl propionate.
In a first variant, said linear ester is ethyl acetate.
In a second variant, said linear ester is methyl butyrate.
In another implementation of the invention, said solvent further comprises a saturated linear carbonate.
Said linear ester is selected form an acetate, a butyrate, a propionate, and mixtures thereof. By way of example, it is possible to select an ethyl acetate, a methyl acetate, a propyl acetate, an ethyl butyrate, a methyl butyrate, a propyl butyrate, an ethyl propionate, a methyl propionate, a propyl priopionate.
The volume proportion of said linear carbonate is not more than 40% of said solvent. When the solvent contains any, the volume proportion of said linear carbonate preferably lies in the range 5% to 40% of said solvent.
In another embodiment of the invention, said solvent further comprises an unsaturated cyclic carbonate.
Said unsaturated cyclic carbonate is selected from vinylene carbonate and derivatives thereof, in particular propylidene carbonate, ethylidene ethylene carbonate, isopropylidene ethylene carbonate. Said unsaturated linear carbonate is preferably vinylene carbonate.
The term xe2x80x9cderivatives of vinylene carbonatexe2x80x9d is used to cover compounds possessing at least one unsaturated bond connected to a carbon atom of the cycle, for example propylidene carbonate, ethylidene ethylene carbonate (or 4-ethylidene 1-3 dioxolane 2 one), or isopropylidene ethylene carbonate (or 4-isopropylidene 1-3 dioxolane 2 one).
The volume proportion of said unsaturated cyclic carbonate is no more than 60% of said solvent. When the solvent contains any, the volume proportion of said unsaturated cyclic carbonate preferably lies in the range 0.5% to 10% of said solvent.
In a first embodiment of the invention, said binder contains an elastomer.
Preferably, said elastomer is selected from a copolymer of acrylonitrile and of butadiene, and a copolymer of styrene and of butadiene.
The proportion by weight of said elastomer lies in the range 30% to 70% of said binder.
In a second embodiment of the invention, said binder contains a cellulose compound.
Preferably, said cellulose compound is a carboxymethyl cellulose having a mean molecular weight greater than about 200,000.
The proportion by weight of said cellulose compound lies in the range 30% to 70% of said binder.
In a third embodiment of the invention, said binder is made up of a mixture of an elastomer and of a cellulose compound.
In a first variant, said binder is made up of a mixture of a copolymer of acrylonitrile and of butadiene, and of carboxymethyl cellulose having a mean molecular weight of greater than about 200,000.
In a second variant, said binder is made up of a mixture of a copolymer of styrene and of butadiene and carboxymethyl cellulose having a mean molecular weight greater than about 200,000.
In the binder, the proportion by weight of said elastomer lies in the range 30% to 70% of said binder and the proportion by weight of said cellulose compound lies in the range 30% to 70% of said binder.
The proportion by weight of said elastomer preferably lies in the range 50% to 70% of said binder and the proportion by weight of said cellulose compound preferably lies in the range 30% to 50% of said binder.
In a fourth embodiment of the invention, said binder contains an acrylic polymer.
Said polymer is preferably a homopolymer of acrylic acid.
The proportion by weight of said acrylic polymer lies in the range 20% to 60% of said binder.
In a fifth embodiment of the invention, said binder is made up of a mixture of an elastomer and of an acrylic polymer.
In a first variant, said binder is made up of a mixture of a copolymer of acrylonitrile and of butadiene, and a homopolymer of acrylic acid.
In a second variant, said binder is made up of a mixture of a copolymer of styrene and of butadiene, and of a homopolymer of acrylic acid.
In the binder, the proportion by weight of said elastomer lies in the range 40% to 80% of said binder and the proportion by weight of said acrylic polymer lies in the range 20% to 60% of said binder.
The cell of the invention has a paste negative electrode comprising a conductive support and an active layer containing the active material and the binder.
The conductive support can be a two-dimensional support, such as a solid or perforated foil, an expanded metal, a grid, or a cloth, or it can be a three-dimensional support such as a felt or a foam having fibers that are metallic, metal-plated, or made of carbon.
The active material is a material suitable for inserting lithium ions at low potential (i.e. not exceeding 1.5 V). The material is preferably selected from carbon in crystal form, such as graphite powder or fibers, graphitizable carbon compounds of low crystal content, such as coke, non-graphitizable carbon compounds of low crystal content, such as vitreous carbon and carbon black, and mixtures thereof.
The cell of the invention has a positive electrode whose active material is a material suitable for inserting lithium ions at high potential (i.e. not less than 2.5 V). This material is preferably selected from a lithiated oxide of a transition metal, such as nickel, cobalt, manganese, vanadium, and iron, a sulfide, a sulfate, and mixtures thereof.
The cell of the invention contains a liquid or solid electrolyte containing a lithium salt. The lithium salt is preferably selected from lithium perchlorate LiClO4, lithium hexafluoroarsenate LiAsF6, lithium hexafluorophoshate LiPF6, lithium tetrafluoroborate LiBF4, lithium trifluoromethanesulfonate LiCF3SO3, lithium trifluoromethanesulfonimide LiN(CF3SO2)2 (LiTFSI), or lithium trifluoromethanesulfonemethide LiC(CF3SO2)3 (LiTFSM).
The present invention also provides the use of a cell of the invention at very low temperatures, i.e. temperatures less than or equal to xe2x88x9220xc2x0 C.
Other characteristics and advantages of the present invention appear from the following examples, naturally given as non-limiting illustrations.